The invention set forth in this specification pertains to new and improved screens such as are commonly utilized for filtration purposes but which are also capable of being utilized in other applications. The invention also pertains to a method of manufacturing such screens.
Screens which are primarily useful as filters have conventionally been manufactured by weaving strands of material into a fabric in which the strands are spaced from one another to a sufficient extent so as to define openings between adjacent strands. On occasion screens useful as filters have been manufactured by knitting one or more strands of material into a knitted fabric in which openings are defined by the positions of the strands relative to the looped or knitted connections between adjacent strands. As polymer technology has advanced it has been increasingly common to form such filter (and other) fabrics out of polymer materials. Although polymer filters consisting of either a knitted or woven fabric are quite utilitarian and widely used in many applications it is considered that these fabrics are not as satisfactory as one might reasonably desire for filtration purposes because of either or both of two different problems.
One of these problems concerns the fact that the individual strands which overlie one another in a woven fabric are, unless the strands are bonded to one another, capable of shifting somewhat relative to one another. If this should occur there would be a change in the size of the openings adjacent to the location where any movement occurred. It is considered that such movement of overlying strands may also be present in connection with knitted fabric filters to a lesser degree than such movement is encountered in connection with woven filters.
Another of these problems concerns the fact that both knitted and woven filter media are of such a character that there is a reasonable chance of the material unnecessarily accumulating on them during their use because of the fact that the strands within them overlie one another so as to create regions where particles may be more or less permanently trapped. Also some material may be more or less permanently attached to the strands because their walls are not continuous and smooth. While in some applications such accumulations along strands or generally at fiber junctions is capable if being beneficial, in others it is considered to be detrimental.
The two problems discussed in the preceding have previously been recognized. As a result of this recognition it has been proposed to bond adjacent, overlying strands of a filter fabric together utilizing either a conventional adhesive or by creating a bond through the use of heat such as can be developed by the application of an ultrasonic horn. These procedures are considered to have their drawbacks. In the former there is always a chance that the adhesive may not adequately connect the overlying strands or may be disadvantageous because it possesses physical properties different from those of the fiber strands. The use of ultrasonic or other heat-type bonding means may be disadvantageous because of the problems of being able to achieve effective bonds and because of the nature of the distortion which is apt to be encountered as a result of the application of heat to at least the surface of a polymer strand. Also expedients of the types indicated are not commonly considered to provide fabric in which the overlying adjacent strands are joined by continuous, smooth surfaces extending generally between and connecting such adjacent strands.
Another problem has been encountered in connection with the use of woven screens for size separation purposes. Because the openings in such screens are normally square the size separation achieved with such screens is not as accurate as may be desired. This is because the distance between the sides of such openings is less than the diagonal distance between the corners of such openings. As a consequence of this there is a tendency for particles which are of less length than such a diagonal distance but longer than the corresponding distance between the sides to either be retained on such screens or to pass them in accordance with how they are oriented as they each such screens or how they may be oriented as a result of movement at the surfaces of such screens.